The arrival of Voyager 2 at Saturn and then Uranus and Neptune had the most important impact on me. I was a teenager at the time, and while I had become interested in astronomy at an early age, the Voyager observations certainly determined my interest in pursuing a career in planetary science.

This false-color view of the rings of Uranus was made by combining images taken by Voyager 2 on 21 Jan. 1986.

Voyager revealed such an amazing planet: Neptune was so blue and had large clouds. I remember making paintings of some of these pictures.

The wealth of images that we obtained from the satellites of the outer planets continue to amaze me.

Uranus and its five major moons are depicted in this montage of images acquired by the Voyager 2 spacecraft.

Miranda, innermost of Uranus' large satellites, is seen at close range in this Voyager 2 image, taken on 24 Jan. 1986.

It is interesting to look at the Voyager pictures, especially now that Galileo and Cassini-Huygens have also explored -- in more detail -- the Jovian and Saturnian systems. We have learned so much over the past fifteen years about the icy satellites in these two systems.

This is a Voyager image of Saturn's moon Enceladus.

Pictured here is a high resolution Cassini image of Enceladus from a close flyby.

Also, the images returned by Voyager 2 of Uranus and Neptune are the only detailed observations available for these planets and their systems. They are thus unique -- literally! However, I am hopeful that we will see a mission to the Uranian and Neptunian systems if not in this decade, then the next one.

In your field of work, what are some examples of the great achievements and discoveries in planetary science and robotic exploration throughout the past 50 years?

NASA's Cassini spacecraft has delivered a glorious view of Saturn, taken while the spacecraft was in Saturn's shadow.

Cassini's extensive tour of the Saturnian system is a paramount example of achievement.

The Cassini spacecraft has revealed a never-before-seen level of detail in Saturn's F ring, including evidence for the perturbing effect of small moonlets orbiting in or close to the ring's bright core.

Through almost a decade of continuous observations, Cassini has provided us with a greater understanding of how giant planet systems work, i.e., how the planet and its fields interact with and shape the satellites, the genetic relationship between satellites and rings, the exchange of material across the system, etc.

In this image (taken by the Cassini spacecraft) we see a quintet of Saturn's moons.

This holistic view of the Saturnian system has revolutionized the way we think about planetary origin, with implications that go beyond our own solar system. We can leverage our knowledge of the Saturnian system to understand how exoplanetary systems get organized and evolve as a consequence of mutual interactions.

Of the countless equinoxes Saturn has seen since the birth of the solar system, this one, captured here in a mosaic of light and dark, is the first witnessed up close by an emissary from Earth ... none other than our faithful robotic explorer, Cassini.

The Cassini-Huygens mission was a particularly well-thought-out mission with a combination of all types of instruments. The Cassini orbiter has been operating in the Saturnian system without any major failures since 2004.

The successful landing of the Huygens probe on Titan was also spectacular.

This mosaic from Cassini-Huygens showing three frames provides unprecedented detail of the high ridge area including the flow down into a major river channel from different sources on Saturn's moon Titan. Image Credit: ESA/NASA/JPL/University of Arizona

Cassini also discovered two astrobiological targets in Enceladus and Titan.

Saturn's rings cut across an eerie scene that is ruled by Titan's luminous crescent and globe-encircling haze, broken by the small moon Enceladus, whose icy jets are dimly visible at its south pole. North is up.

Incidentally, the Cassini-Huygens mission was also a great success on the political front, with 18 countries working together towards achieving the same goal.

Another great achievement is the progress in the development of ion propulsion.

An artist's rendition of the Dawn spacecraft orbiting Ceres.

We are going through a big change in the way missions are designed thanks to the increasing performance of this technology. The Dawn mission is a fantastic example of ion propulsion and how it facilitates having multiple destinations. Ion propulsion also enables extensive coverage of planetary bodies through "rendezvous."

NASA's Dawn spacecraft obtained this image with its framing camera on 18 July 2011. It was taken from a distance of about 10,500 km (6,500 miles) away from the protoplanet Vesta. The smallest detail visible is about 2.0 km (1.2 miles).

Dawn's visit to Vesta was impressive, but it is the prospect of its rendezvous with asteroid Ceres, Vesta's water-rich counterpart, that will greatly increase the overall science return of this mission by enabling the scientists to compare these two protoplanets.